Kantar Media/CMAG monitors TV signals for political advertising nationwide, capturing ads each time they run. Then, using a proprietary formula, it estimates how much placing each ad costs.

Like any estimate, it's imperfect. Here's what it covers, and what it doesn't:

Just placement on TV — The estimate only covers TV ads, not other kinds of political messages, such as ads that appear on radio or online. The estimate also only includes how much money a candidate or organization spent
to place the ad, not to make it.

No local cable — Kantar Media monitors local broadcast TV in all 210 media markets, as well as national network and national cable TV advertising. If an ad runs on a local cable channel, it won't be counted here.

Any political ad — Unlike records filed at the Federal Elections Commission, this information includes so-called “issue ads” that mention a Senate candidate but don’t overtly call for the candidate’s election or defeat.
Unless run immediately before a primary or general election, issue ad spending does not have to be reported to the FEC.

No future ads — Unlike some records from the Federal Communications Commission, it only counts ads that have already run. Future ad buys are not included.

Subject to dispute — Since the estimate is based on a formula, it may not exactly reflect what placing the ad actually cost. Think of the cost estimate as a well-informed guess, which can provide useful points of comparison.